Composition for chromium plating a substrate and chromium plating process using such a composition

ABSTRACT

An aqueous composition for the electrolytic deposition of a chromium coating on the surface of a substrate is disclosed, in which it has a pH of between 0 and 1 and which contains a trivalent chromium salt, glycine, an alkali metal salt, an aluminium salt and, optionally, an ammonium salt. The chromium plating process according to the invention comprises the immersion of the substrate to be treated in this composition and the application of a current between this substrate and an anode. This process makes it possible to form a high-quality chromium coating on the entire surface of the substrate.

The present invention lies in the field of the electrochemical deposition of chromium on the surface of a substrate, with a view to improving the resistance to wear thereof, such a deposition normally being referred to by the term “chromium plating”.

More particularly, the present invention relates to an aqueous composition for the electrolytic deposition of a chromium coating on the surface of a substrate, as well as a method for the electrolytic deposition of a chromium coating on the surface of a substrate using such a composition. The invention also relates to a substrate covered on the surface with a chromium coating obtained by such a method.

The electrolytic deposition of a surface chromium coating on a substrate, also referred to as chromium plating, is used in many fields, in particular in the aeronautical, automobile, mechanical, etc. fields, for improving the resistance to wear of the parts (hard chromium plating is then spoken of), or for producing decorative deposits (decorative chromium plating is then spoken of), the thickness of the surface coating varying from a few tenths of microns for decorative chromium plating to a few hundreds of microns for hard chromium plating. Hard chromium plating, with which the present invention is particularly concerned, makes it possible in particular to reduce wear on the parts during their relative movements as well as the coefficient of friction thereof.

In order to obtain a chromium coating on the surface of a substrate, chromium trioxide (CrO₃) is generally used, a substance based on hexavalent chromium which, in aqueous solution, generates chromic acid. Hard chromium is obtained by electrolytic reduction of the hexavalent chromium into metallic chromium on the surface of the part to be treated. However, since substances based on hexavalent chromium are toxic for living organisms, for the past several years it has been sought to limit or even completely eliminate use thereof.

Several technologies have been proposed by the prior art for replacing current hard chromium plating methods based on hexavalent chromium.

Among these technologies, mention can be made of thermal projection of the HVOF type (High Velocity Oxygen Fuel—projection by supersonic flame), which is interesting but does not appear to be sufficiently flexible in application, and which is in particular not suitable for treating parts with a complex shape. This technology furthermore proves to have high cost: the application thereof would be two to ten times more expensive than hard chromium plating with hexavalent chromium.

So-called dry methods, such as physical vapour deposition or chemical vapour deposition, have also been proposed and are used industrially. However, the operating constraints related to these technologies, and the consequences that result therefrom, limit the application thereof on a large scale. Furthermore, these methods also do not make it possible to produce deposits the properties of which are satisfactory over the entire surface of the part when they are applied to parts with a complex shape.

Electrolytic or chemical deposition methods, making it possible in particular to solve the difficulties introduced by complex geometric surfaces, have furthermore been proposed by the prior art. These methods generally use aqueous solutions of nickel or cobalt salts to make metallic depositions, with or without inclusions of particles such as hard, self-lubricating, etc. particles, on the surface of the part to be treated. However, the use of cobalt salts, just like the use of nickel salts, is undesirable, because of a risk of toxicity for living organisms.

Seeking to develop an alternative to hard chromium plating methods based on hexavalent chromium, the present inventors have taken a particular interest in electrolytic methods using trivalent chromium in aqueous solution.

Such methods have been proposed by the prior art for chromium plating metal parts. By way of illustration of such prior art, mention can be made in particular of the documents U.S. Pat. No. 4,142,948, US 2009/0211914 or WO 2015/110627, which all describe chromium plating methods using an aqueous solution based on trivalent chromium.

However, these methods of the prior art all have drawbacks. In particular, either they relate more to decorative chromium plating rather than hard chromium plating, i.e. the wear-resistance properties of the chromium coatings that they make it possible to form on the parts are not sufficiently high for the applications concerned by hard chromium plating, or they use toxic substances, or they function over a current density range that is too limited, or they use compositions the service life of which in operation is insufficient for industrial use, or even they combine a plurality of these drawbacks.

The current density range in which the chromium plating method can function has in particular great importance, in particular when the parts to be treated have a complex shape. Indeed, during an electrolytic deposition, it is observed that, for a particular current density imposed in the electrolytic bath, the actual current density on the surface of the part being treated may vary locally to a very great extent. In order to guarantee a homogeneous deposition of chromium on the whole of a part, it is therefore important to have available a method that can function with a current density range as wide as possible, this proving all the more important that the shape of the part is more complex.

The document U.S. Pat. No. 4,142,948 for example describes aqueous compositions based on trivalent chromium which either function over limited current density ranges or have an insufficient service life.

The publications by Suvegh et al., 1998, in Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 455: 69-73 and by EI-Sharif et al., 1995, in Transactions of the Institute of Metal Finishing, 73: 19-25, as well as the document CN 105 706 513, all also describe aqueous compositions based on trivalent chromium for the electrolytic deposition of a chromium coating on the surface of a substrate. These compositions also have an insufficient service life in operation.

The present invention aims to remedy the drawbacks of the methods proposed by the prior art for hard chromium plating without hexavalent chromium, in particular the drawbacks disclosed above, by proposing a method, and a composition for implementation thereof, that make it possible, without using a substance that is toxic for living organisms, and in particular hexavalent chromium, to form on a substrate a chromium coating of good quality, having in particular resistance to wear that is at least equivalent to that obtained by the hard chromium plating methods of the prior art using hexavalent chromium, this coating being formed on the entire surface thereof, and this whatever the shape of the substrate, including for substrates with a complex shape. The invention also aims for this composition to remain effective over time.

An additional objective of the invention is that the cost of this composition and of the implementation of this method should be as low as possible, such implementation moreover being easy.

It has been discovered by the present inventors that these objectives can be achieved by using, as an electrolytic bath, an aqueous composition based on trivalent chromium with a particular constitution, as to the selection of the constituents thereof and the selection of their respective concentrations, and the pH of which is specifically between 0 and 1.

Thus, according to a first aspect, an aqueous liquid composition for the electrolytic deposition of a chromium coating on the surface of a substrate is proposed according to the present invention, this electrolytic deposition also being designated, in the present description, by the term “chromium plating”.

The aqueous composition according to the invention contains:

-   -   0.4 to 0.9 mol/l of a trivalent chromium salt,     -   0.6 to 0.9 mol/l of glycine,     -   an alkali metal salt,     -   an aluminium salt,     -   and optionally an ammonium salt.

The pH thereof is between 0 and 1.

The aqueous composition according to the invention is furthermore preferably essentially devoid of hexavalent chromium. What is meant by this is that the composition does not contain hexavalent chromium, or only in the trace state.

In the present description, conventionally per se, trivalent chromium means chromium in the +3 oxidation state, and hexavalent chromium means chromium in the +6 oxidation state.

The concentration of glycine in the composition may in particular be between 0.63 and 0.9 mol/l, in particular between 0.63 and 0.83 mol/l.

In alternative embodiments of the invention, in particular especially advantageous when the composition does not include any ammonium salt, the glycine concentration in the composition is between 0.6 and 2.8 mol/l, in particular between 0.63 and 2.8 mol/l, and for example between 0.67 and 2.8 mol/l.

The aqueous composition according to the invention, used as an electrolytic bath in a chromium plating method, makes it possible to obtain, on the surface of the substrate being treated, a high-quality chromium coating, and this over the whole of this surface, including when the substrate has a complex shape.

Such an advantageous result is in particular due to the fact that this aqueous composition allows effective operation of the chromium plating method that implements it over a very wide current density range, with an amplitude as high as 72 A/dm², and which may even be greater than 84 A/dm². The chromium plating method using the composition according to the invention can thus function, in order to form a high-quality chromium coating on the surface of the substrate being treated, in a current density range of up to more than 100 A/dm², in particular as wide as ranging from 13 A/dm² to more than 100 A/dm².

The chromium coating that can be formed on the surface of the treated substrate by means of the aqueous composition according to the invention advantageously has the following properties:

-   -   it has a Vickers hardness greater than 800 Hv, measured in a way         that is conventional per se, for an applied load of 100 g;     -   it exhibits good adhesion to the substrate. In particular, no         detachment of the coating is observed when the substrate treated         is subjected to the grinding test in accordance with ASTM B571         standard; also, no detachment of the coating is observed         following thermal shock, in particular after the treated         substrate has been subjected to three cycles each comprising         heating the coated substrate at 300° C. for one to two hours and         then rapid cooling it to 20° C. by immersion in cold water. Such         a property advantageously enables the coating to resist well the         high stresses to which it is subjected during uses of the         chromium-plated substrate in severe conditions, for example when         this coating is formed on the surface of actuator cylinders,         brake pistons, etc.;     -   it has high resistance to rectification. In particular, no         detachment of the coating is observed after rectification of         thickness greater than 100 μm;     -   it has high resistance to wear. In this regard, the performance         thereof, measured by the Taber test, in accordance with ASTM         D4060 standard, is in particular equivalent to that obtained for         coatings obtained by methods using hexavalent chrome of the         prior art;     -   it does not degrade, or degrades only very little, the         mechanical properties of the substrates treated. For example,         subjected to the hydrogen weakening test, the substrates treated         being under traction, in accordance with ASTM F519 standard, no         rupture is observed after 200 hours at 75% of the breaking         point;     -   it exhibits good adhesion to undercoats, in particular to         undercoats with anticorrosion properties, especially nickel         undercoats, which may have been formed beforehand on the         substrate.

As indicated above, these properties are advantageously uniform over the entire surface of the substrate, even when it has a complex shape.

It has furthermore been discovered by the present inventors that, when it is used for the electrolytic deposition of chromium on the surface of a substrate, entirely surprisingly, the service life of the aqueous composition according to the invention, having a pH of between 0 and 1, and a glycine concentration of between 0.6 and 0.9 mol/l, is significantly greater than for equivalent aqueous compositions, i.e. containing the same components in the same concentrations, but the pH of which is greater than 1 and/or the glycine concentration of which is greater than 0.9 mol/l. The aqueous composition according to the invention can thus advantageously be used, for the electrolytic deposition of chromium on the surface of a substrate, after having imposed therein the equivalent of 22 Ah/L, and even more than 50 Ah/L for certain embodiments, without significant loss of quality of the deposition produced, this without having added in the composition more of one or several of the constituents thereof. Such an advantageous result makes it possible to predict, for industrial implementation, wherein readjustments of the concentrations of the constituents of the aqueous composition used will be made regularly, a service life of the composition that is considerably higher and particularly important for the field of hard chromium plating.

The pH of the aqueous composition according to the invention can be adjusted to a value of between 0 and 1 according to any method that is conventional per se for a person skilled in the art, in particular by the addition of acid, for example hydrochloric acid, in said composition.

Thus, the aqueous composition according to the invention may in particular contain one or more acids, for example hydrochloric acid, in a suitable quantity for conferring a pH of between 0 and 1 on said aqueous composition.

Preferentially, the pH of the aqueous composition according to the invention is substantially equal to 0.5.

The aqueous composition according to the invention is easy to prepare, by simple mixing of the constituents thereof in water, and has a low-cost price.

The aqueous composition according to the invention can furthermore meet one or more of the characteristics described below, implemented individually or in each of the technically operative combinations thereof.

For all the components of the composition, the preferential concentration values indicated below are all associated with better still performance of the chromium plating method using the composition according to the invention, in particular in terms of current density range wherein this method functions effectively. The tight concentration ranges indicated below are thus associated with current density ranges wherein the chromium plating method is effective, which are wider.

The glycine concentration in the composition is for example between 0.67 and 0.83 mol/l. Optimally, the glycine concentration in the aqueous composition according to the invention may be approximately equal to 0.75 mol/l.

In the present description, trivalent chromium salt means a single trivalent chromium salt or a mixture of various trivalent chromium salts.

The trivalent chromium salt contained in the aqueous composition according to the invention may contain, in addition to the Cr³⁺ ion, any counterion that is conventional per se for chromium plating treatments, or any mixture of such counterions.

The trivalent chromium salt may in particular be selected from the group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, which may be organic or inorganic, sulfonates, which may be organic or inorganic, thiocyanates, or any one of the mixtures thereof.

More generally, in the composition according to the invention, at least one, preferably several, and preferentially all, of the trivalent chromium salt, the alkali metal salt, the aluminium salt and, where applicable, the ammonium salt, is/are selected from chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, which may be organic or inorganic, sulfonates, which may be organic or inorganic, thiocyanates, or any one of the mixtures thereof.

Each of the salts forming part of the aqueous composition according to the invention may include a single counterion, or a mixture of a plurality of counterions. Preferentially, each salt is formed with a single counterion.

In preferred embodiments of the invention, in the aqueous composition, the counterion or counterions of the trivalent chromium salt, the counterion or counterions of the alkali metal salt, the counterion or counterions of the aluminium salt, and where applicable the counterion or counterions of the ammonium salt, are identical.

In the above particular embodiments thereof, the aqueous composition according to the invention is then advantageously more stable over time.

The trivalent chromium salt may for example be selected from the following trivalent chromium salts: CrCl₃.xH₂O, (CH₃CO₂)₂Cr.xH₂O, (CH₃CO₂)₇Cr₃(OH)₂.xH₂O, CrF₃.xH₂O, etc.

Preferentially, the trivalent chromium salt is a chromium chloride, for example CrCl₃.6H₂O.

The concentration of trivalent chromium salt in the composition is for example between 0.41 and 0.86 mol/l.

It is preferably between 0.7 and 0.9 mol/l, for example between 0.71 and 0.86 mol/l. Optimally, the concentration of trivalent chromium salt in the aqueous composition according to the invention may be approximately equal to 0.79 mol/l.

In the present description, aluminium salt means a single aluminium salt or a mixture of various aluminium salts.

The aluminium salt contained in the aqueous composition according to the invention may contain, in addition to the aluminium ion, any counterion conventional per se for chromium-plating treatments, or any mixture of such counterions, in particular one or more of the counterions listed above.

For example, the aluminium salt is an aluminium chloride AlCl₃.

The concentration of aluminium salt in the aqueous composition according to the invention is preferably between 0.06 and 0.7 mol/l, for example between 0.06 and 0.62 mol/l.

It is preferably between 0.2 and 0.3 mol/l, for example between 0.23 and 0.29 mol/l. Optimally, the concentration of aluminium salt in the aqueous composition according to the invention may be approximately equal to 0.26 mol/l.

In the present description, alkali metal salt means a single alkali metal salt or a mixture of various alkali metal salts.

The alkali metal is preferably sodium or potassium, or one of the mixtures thereof.

The alkali metal salt contained in the aqueous composition according to the invention may contain, in addition to the ion of the alkali metal, any counterion conventional per se for chromium-plating treatments, or any mixture of such counterions, in particular one or more of the counterions listed above.

For example, the alkali metal salt is a sodium chloride NaCl and/or a potassium chloride KCl.

The concentration of alkali metal salt in the aqueous composition according to the invention is preferably between 0.2 and 1.9 mol/l, for example between 0.26 and 1.88 mol/l.

In the present description, ammonium salt means a single ammonium salt or a mixture of various ammonium salts.

The ammonium salt contained in the aqueous composition according to the invention may contain, in addition to the ammonium ion, any counterion conventional per se for chromium-plated treatments, or any mixture of such counterions, and in particular one or more of the counterions listed above.

For example, the ammonium salt is ammonium chloride NH₄Cl.

The concentration of ammonium salt in the aqueous composition according to the invention is preferably between 0 and 1.0 mol/l, for example between 0 and 0.93 mol/l.

In variants of the invention, the aqueous composition contains:

-   -   a concentration of ammonium salt of between 0.5 and 0.8 mol/l,         for example between 0.58 and 0.72 mol/l; and preferentially         approximately equal to 0.65 mol/l,     -   and a concentration of alkali metal salt of between 0.2 and 1.3         mol/l, for example between 0.26 and 1.28 mol/l, and preferably         between 0.5 and 0.7 mol/l, for example between 0.54 and 0.66         mol/l; and preferentially approximately equal to 0.60 mol/l.

In different variants of the invention, the aqueous composition is devoid of ammonium salt, and contains a concentration of alkali metal salt of between 1.5 and 1.9 mol/l, for example between 1.54 and 1.88 mol/l.

It has been found by the present inventors that, surprisingly, each of these variants of the aqueous composition according to the invention, used in a chromium-plated method, enables this method to function, by forming on the surface of the substrate a high-quality chromium coating, within a very wide current density range, even for compositions devoid of ammonium salt.

A particular aqueous composition according to the invention contains:

-   -   0.41 to 0.86 mol/l, preferably 0.71 to 0.86 mol/l, of trivalent         chromium salt, for example trivalent chromium chloride;     -   0.63 to 0.9 mol/l, preferably 0.63 to 0.83 mol/l and         preferentially 0.67 to 0.83 mol/l, of glycine;     -   0.26 to 1.28 mol/l, preferably 0.54 to 0.66 mol/l, of alkali         metal salt, for example sodium chloride;     -   0.01 to 0.93 mol/l, preferably 0.58 to 0.72 mol/l, of ammonium         salt, for example ammonium chloride;     -   and 0.06 to 0.62 mol/l, preferably 0.23 to 0.29 mol/l, of         aluminium salt, for example aluminium chloride;     -   the pH thereof being between 0 and 1.

Another particular aqueous composition according to the invention contains:

-   -   0.41 to 0.86 mol/l, preferably 0.71 to 0.86 mol/l, of trivalent         chromium salt, for example trivalent chromium chloride;     -   0.63 to 2.80 mol/l, preferably 0.63 to 0.9 mol/l, preferably         0.63 to 0.83 mol/l and preferentially 0.67 to 0.83 mol/l, of         glycine;     -   1.54 to 1.88 mol/l of alkali metal salt, for example sodium         chloride;     -   and 0.06 to 0.62 mol/l, preferably 0.23 to 0.29 mol/l, of         aluminium salt, for example aluminium chloride;     -   the pH thereof being between 0 and 1;     -   and the composition being devoid of ammonium salt.

The aqueous composition according to the invention may, optionally, contain substances other than those listed above, to the exclusion of hexavalent chromium, but these substances should however not interfere with the action of the essential constituents of the aqueous composition listed above, for the electrolytic deposition of chromium on the surface of the substrate.

By way of non-limitative example of the invention, the aqueous composition according to the invention may for example contain one or more surfactants.

Preferentially, the aqueous composition according to the invention is substantially devoid of one or more, preferably all, of the following substances: boric acid/borate, or another compound based on boron, quaternary ammonium, oxalate, vanadium, manganese, iron, cobalt, molybdenum, nickel, tungsten and indium.

Substantially devoid herein means that the aqueous composition does not contain these substances, except in the trace state, i.e., in a non-operative quantity.

According to another aspect, the present invention relates to a method for the electrolytic deposition of a chromium coating on the surface of a substrate, referred to as a chromium plating method. This method comprises:

-   -   immersing, in a bath, referred to as an electrolytic bath, of an         aqueous composition according to the invention, complying with         one or more of the above characteristics, the substrate and an         anode,     -   and applying a current between the substrate, which then         constitutes a cathode, and the anode.

The electrolytic reduction of the trivalent chromium into hard metallic chromium occurs in the bath, on the surface of the part to be treated. The chromium coating thus formed by the method according to the invention on the surface of the substrate has the entirely advantageous properties disclosed above.

The current applied between the substrate and the anode may be of the pulsed type. In preferred embodiments of the invention, a continuous current is applied between the substrate and the anode.

Such a feature advantageously makes it possible to be free from the difficulties of use of pulsed currents on an industrial scale, and to simplify the implementation of the chromium plating method according to the invention.

The method according to the invention is in particular entirely adapted to industrial use, in a simple way.

In preferred embodiments of the invention, the continuous current density imposed between the substrate and the anode is between 10 and 100 A/dm², preferably between 20 and 40 A/dm², and preferentially approximately equal to 30 A/dm².

As for the temperature of the electrolytic bath, this is preferably between 20 and 80° C., more preferably between 20 and 60° C., and preferentially between 40 and 60° C. It is for example approximately 45° C. or 50° C.

The applying of a current between the substrate and the anode is carried out for a duration that is suitable for forming on the surface of the substrate a chromium coating with a thickness of between 5 and 500 μm. It is within the skills of a person skilled in the art to be able to choose this duration, according in particular to other operating parameters, in particular the temperature of the aqueous composition, the exact composition thereof and the current density applied.

For example, for a set of given operating parameters, a person skilled in the art will for this purpose be able to test a plurality of periods of application of the current, and then measure, for each sample substrate treated, the thickness of the chromium coating formed on the surface thereof, for example by electron microscopy, and to deduce therefrom the appropriate duration of application of current corresponding to these operating parameters and to the thickness of the chromium coating sought.

The substrate to which the chromium plating method according to the invention is applied is formed from metal material or any other material having an electrically conductive surface.

In this regard, it may be formed from any metal. The chromium plating method according to the invention proves to be particularly advantageous for implementation on steel substrates. In the present description, the term steel includes steel alloys, in particular stainless-steel alloys. The chromium plating method according to the invention may also for example be implemented on substrates made from nickel-based superalloy, cobalt-based superalloy, bronze, aluminium alloy, magnesium alloy, titanium alloy, etc.

Prior to the implementation of the chromium plating method according to the invention, the substrate may have been covered with one or more undercoats, for example an undercoat of nickel, by any method conventional per se.

The anode used in the chromium plating method according to the invention can be formed from any material conventional per se for the electrolytic deposition of metal, in particular of chromium, on a substrate. It may for example be formed from an inert conductive material such as graphite, iridium-titanium, platinised titanium or titanium covered with a metal oxide mixture (MMO) or any other conductive material covered with one of these materials.

The method according to the invention may comprise prior steps of degreasing, in particular alkaline degreasing, and/or pickling, of the substrate.

Such degreasing and pickling steps may be implemented in accordance with any method conventional per se for a person skilled in the art.

Preferentially, the method according to the invention comprises a step of alkaline degreasing of the substrate, by putting the substrate in contact, in particular by immersion, in an alkaline composition, such as the composition sold under the name Presol 7045 by the company Coventya. This contacting is for example implemented for a period of 20 minutes, the composition being at a temperature of approximately 60° C.

This prior degreasing step may be followed by a pickling step.

A step of pickling the substrate may in particular consist of an electrolytic pickling in a composition based on sulfuric acid, for example containing a mixture of sulfuric acid and ethylene glycol. This pickling step may be implemented in a conventional manner, for example be implemented at ambient temperature, i.e., at a temperature of approximately 20° C., by applying for example, for the anodic phase, a current density of 40 A/dm² for 45 seconds and, for the cathodic phase, a current density of 30 A/dm² for four minutes.

The method according to the invention may furthermore comprise final steps of:

-   -   rinsing the substrate, in particular with water, and optionally         drying     -   and/or degassing, for example at 190° C. for 3 hours.

The chromium plating method according to the invention may also comprise a step of heat treatment of the substrate covered with a chromium coating obtained, for example at a temperature of between 250° C. and 700° C. for a period of between 20 and 200 minutes. Such a heat treatment step makes it possible in particular to increase the hardness of the chromium coating covering the substrate, by a phenomenon of structural hardening.

Another aspect of the invention relates to a substrate, in particular a metal substrate or one having an electrically conductive surface, for example a steel substrate, obtained by a chromium plating method according to the invention.

This substrate is covered on the surface with a chromium coating with a thickness of between 5 and 500 μm, in particular between 15 and 450 μm. This coating has characteristics equivalent to those of the chromium coatings formed by the chromium plating methods of the prior art using hexavalent chromium, in particular in terms of hardness, coefficient of friction and resistance to wear. It has in particular a Vickers hardness, measured for an applied load of 100 g, which is greater than 800 Hv after degassing at 190° C. for 3 hours, and which is even greater than 1200 Hv after heat treatment at 300° C. for 120 minutes.

This substrate may be any mechanical part, including a part with a complex shape.

Its properties, in particular of resistance to wear, make it entirely adapted to use under high stresses, in any industrial field, in particular in the aeronautical field.

The features and advantages of the invention will emerge more clearly in the light of the example embodiments below, provided simply for illustration and in no way limitative of the invention, with the support of FIGS. 1 to 4, wherein:

FIG. 1 shows a photograph of steel substrates treated by a chromium plating method according to the invention, the aqueous composition used having a pH of 0.5 and being at various ageing stages (expressed in Ah/L), the lower part of the substrates treated having been rubbed with abrasive paper;

FIG. 2 shows a photograph of steel substrates treated by a chromium plated method according to the invention, the aqueous composition used having a pH of 1 and being at various stages of ageing (expressed in Ah/L), the lower part of the treated substrates having been rubbed with abrasive paper;

FIG. 3 shows a photograph of a substrate partially covered with a chromium coating at the end of a Hull cell test using an aqueous composition according to the invention, the associated current density values varying between 0 A/dm² (on the right in the figure) and more than 100 A/dm² (on the left in the figure), a chromium coating being observed for current densities greater than or equal to 13 A/dm²;

and FIG. 4 shows photographs of steel substrates treated by a chromium plating method, by means respectively of an aqueous composition comprising a concentration of glycine of 0.75 mol/l (a/), an aqueous composition comprising a glycine concentration of 1 mol/l (b/) and an aqueous composition comprising a glycine concentration of 1.25 mol/l (c/), each aqueous composition used being at various ageing stages (expressed in Ah/L), and the lower part of the substrates treated having been rubbed with abrasive paper.

EXAMPLE 1

Cylindrical substrates made from XC38 steel, of 20 mm in diameter and 200 mm long, are subjected to the following steps of a chromium plating method according to the invention:

1/ Alkaline degreasing, by immersing the substrate in a Presol 7045 composition from Coventya at a temperature of 60° C. for 20 min.

2/ Electrolytic pickling in a sulfuric medium, by immersing the substrate in a composition of sulfuric acid and ethylene glycol at ambient temperature, applying for the anodic phase a current density of 40 A/dm² for 45 s and for the cathodic phase a current density of 30 A/dm² for 4 min.

3/ Hard chromium plating

For this purpose, the substrate is immersed, with an iridium-titanium anode, in a bath of an aqueous composition according to the invention, containing, in solution in water:

-   -   0.79 mol/l of CrCl₃.6H₂O     -   0.75 mol/l of glycine     -   0.60 mol/l of NaCl     -   0.65 mol/l of NH4Cl     -   0.26 mol/l of AlCl₃

The pH of this aqueous composition has been previously adjusted to a value of 0.5 by adding a suitable quantity of hydrochloric acid in the composition.

The temperature of the aqueous composition is 45° C.

Various substrates are treated successively in a bath of this aqueous composition.

For each substrate, a current density of 40 A/dm² is imposed between the substrate and the anode for a suitable duration for forming on the surface of the substrate a chromium coating with a thickness of 50 nm, which for each coating corresponds in this precise case to a quantity of electrical load imposed per volume of aqueous composition of between 2.2 and 2.3 Ah/L. Several substrates are thus treated successively in this same bath at various stages of ageing, until a bath ageing of 38.1 Ah/L is reached.

After this treatment, each substrate is subjected to a degassing step for 3 h at 190° C.

At the end of this method, for each substrate treated, a chromium coating of uniform thickness is obtained on the surface of the substrate, this coating being of metal grey colour, homogeneous, devoid of any black marks that would testify to the presence of chromium oxides instead of metallic chromium. These observations testify to a good quality of the chromium coating formed on the surface of the substrate.

For each substrate treated, the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.

FIG. 1 shows a photograph of the substrates thus obtained. In this figure, the values expressed in Ah/L associated with each substrate correspond to the stages of ageing of the aqueous composition at the end of the treatment of this substrate, the various substrates having been successively treated in the composition.

As can be clearly observed, no change in appearance has occurred at the lower parts of the substrates that have been rubbed. The metallic coatings are all adherent to the substrate, even when they have been formed in an electrolytic bath wherein the equivalent of 38.1 Ah/L has been imposed, and this without having added in the bath additional quantities of one or several of the constituents thereof.

This demonstrates that the electrolytic bath according to the invention has a long service life, and that the chromium plating method makes it possible to form on the surface of the substrate a metallic chromium coating having good adhesion.

The other properties of this chromium coating are evaluated as follows, for each of the substrates treated.

The thickness of the coating, measured by electron microscopy, is between 5 and 500 μm.

The Vickers hardness, measured for an applied load of 100 g, is greater than 800 Hv.

No detachment of the coating is observed when the substrate is subjected to the grinding test in accordance with ASTM B571.

Also, no detachment of the coating is observed following a thermal shock, in particular after the substrate has been subjected to three cycles comprising heating at 300° C. for 1 to 2 h and then cooling by immersion in cold water.

No detachment of the coating is observed after rectification by a thickness greater than 100 μm.

The resistance to wear of a coating, measured by the Taber test, in accordance with ASTM D4060 standard, is equivalent to that obtained for the coatings obtained by the methods using hexavalent chrome of the prior art.

Subjected to the hydrogen weakening test, the substrate treated being under traction, in accordance with ASTM F519 standard, no rupture is observed after 200 hours at 75% of the breaking point.

All these properties testify to a particularly high performance of the chromium plating method according to the invention.

EXAMPLE 2

In this example, substrates made from XC38 steel identical to those described in Example 1 are treated, in accordance with the present invention, as indicated in Example 1, except that the pH of the aqueous composition used was adjusted to a value of 1, by adding hydrochloric acid in the composition.

At the end of this method, a chromium coating of uniform thickness is obtained on the surface of the substrate, this coating being metal grey in colour, homogeneous, devoid of any black marks.

For each substrate treated, the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.

FIG. 2 shows a photograph of the substrates thus obtained. In this figure, the values expressed in Ah/L associated with each substrate correspond to the stages of ageing of the aqueous composition at the end of the treatment of this substrate, the various substrates having been successively treated in the composition.

As can be clearly observed, the metallic coatings are all adherent to the substrate even when they have been formed in an electrolytic bath wherein the equivalent of 22.4 Ah/L has been imposed, and this without having added in the bath additional quantities of one or more of its constituents. A loss of adhesion is found afterwards, as shown by the white arrows, which designate the regions rubbed with abrasive paper. These results, even if they are less good than those obtained with the electrolytic bath of pH 0.5 of Example 1, are all the same very satisfactory with regard to the service life of the bath.

The properties of the coatings formed on the substrates, at least up to 22.4 Ah/L, are similar to those described above for the coatings of Example 1.

EXAMPLE 3

A method according to the invention is implemented in accordance with the conditions described in Example 1, for a substrate as described in Example 1.

The current density imposed between the substrate and the anode is 40 A/dm² for 40 min.

At the end of this method a chromium coating is obtained with a thickness of 50 μm on the surface of the substrate.

This coating has:

-   -   a Vickers hardness (measured with a load of 100 g) of 900 Hv         after degassing at 190° C. for 3 h, and of 1300 Hv after such a         degassing and then a heat treatment of 300° C. for 2 h;     -   a wear index, measured by the Taber test, in accordance with         ASTM D4060 standard (grinder: CS-10, load: 1000 g on each arm),         equal to 3.

The performances indicated above in Example 1, in terms of adhesion to the substrate and absence of negative effect on the mechanical properties of the substrate, are also achieved.

EXAMPLE 4

Various aqueous compositions according to the invention (named C1 to C13) or not in accordance with the invention (named C14 to C20) are tested in order to evaluate the current density range in which a chromium plating method using them can function.

For this purpose, a Hull cell study is carried out so as to define the current density range corresponding to each composition tested.

The Hull cell has a trapezoidal shape and makes it possible to position the cathode and the anode, constituting opposite walls of the cell, in a way that is not parallel to each other. The other two walls are parallel and insulating. By making it possible to obtain a wide variation in current density on the cathode surface, this cell makes it possible to evaluate the influence of the latter on the quality of the deposition of chromium.

For these tests, brass plates (substrates) are used at the cathode and an iridium-titanium grid is used at the anode. The deposit of chromium having a grey colour and brass being golden, it is possible to visually estimate the current density range wherein the deposition can form.

For these tests, the temperature of the composition is 45° C., and a current density of 8 A is applied for 1 min 30 s.

The current density applied at a precise point of the cathode can be determined by means of the following formula:

j=I×(5.10-5.24 log d)

-   -   wherein     -   j represents the current density in A/dm² at the point in         question     -   I represents the current density in A passing through the cell     -   d represents the distance in cm between the origin of the         cathode and the point in question, the origin of the cathode         corresponding to the end of the cathode closest to the anode.

For the aqueous composition according to the invention described in Example 1, here named C1, extreme current density values are thus for example determined, for the operating current density range associated with the composition, which are equal to 13 A/dm² for the low value and greater than 100 A/dm² for the high value.

Between these extreme current density values, a chromium coating of entirely satisfactory quality is obtained on the substrate, as shown in FIG. 3, this coating being metal grey in colour, homogeneous and devoid of any black marks.

The results obtained by the Hull cell test, for the various aqueous compositions tested, are indicated in Table 1 below.

TABLE 1 results of the Hull cell test for compositions according to the invention (C1 to C13) and other compositions (C14 to C20) Current Composition density CrCl₃ Glycine NaCl NH₄Cl AlCl₃ (A/dm²) No (mol/l) (mol/l) (mol/l) (mol/l) (mol/l) pH Max. Min. C1 0.79 0.75 0.60 0.65 0.26 0.5 >100 13 C2 0.41 0.75 0.60 0.65 0.26 0.5 >100 16 C3 0.86 0.75 0.60 0.65 0.26 0.5 >100 16 C4 0.79 0.63 0.60 0.65 0.26 0.5 >100 20 C5 0.79 2.80 0.60 0.65 0.26 0.5 >100 28 C6 0.79 0.75 0.26 0.65 0.26 0.5 >100 24 C7 0.79 0.75 1.28 0.65 0.26 0.5 >100 15 C8 0.79 0.75 0.60 0.00 0.26 0.5 >100 18 C9 0.79 0.75 0.60 0.93 0.26 0.5 >100 15 C10 0.79 0.75 0.60 0.65 0.06 0.5 >100 15 C11 0.79 0.75 0.60 0.65 0.62 0.5 >100 18 C12 0.79 0.75 0.60 0.65 0.26 0 >100 28 C13 0.79 0.75 0.60 0.65 0.26 1.0 >100 24 C14 0.79 0.75 0.60 0.65 0.26 1.5 35 18 C15 0.79 0.75 0.60 0.65 0.00 0.5 60 15 C16 0.79 0.75 0.00 0.65 0.26 0.5 100 40 C17 0.79 3.33 0.60 0.65 0.26 0.5 100 50 C18 0.34 0.75 0.60 0.65 0.26 0.5 70 16 C19 0.94 0.75 0.60 0.65 0.26 0.5 60 24 C20 0.79 0.47 0.60 0.65 0.26 0.5 60 24

These results clearly show that the compositions according to the present invention (C1 to C13) are all associated with very wide current density ranges, the composition with the highest performance being composition C1.

The current density ranges determined for the compositions that are not in accordance with the present invention (C14 to C20) are, in comparison, much narrower.

In particular, the results obtained, in terms of amplitude of the current density range, are significantly inferior when the pH of the aqueous composition is greater than 1, than when the pH is between 0 and 1 as recommended by the present invention.

A Hull cell test is also carried out, under the operating conditions described above, but with a temperature of the composition of 50° C. or 55° C., for a composition according to the invention containing, in solution in water:

-   -   0.79 mol/l of CrCl₃.6H₂O     -   0.75 mol/l of glycine     -   1.71 mol/l of NaCl     -   et 0.26 mol/l of AlCl₃.

This solution is devoid of ammonium salt. The pH thereof has previously been adjusted to a value of 0.5 by adding a suitable quantity of hydrochloric acid in the composition.

The following operating current density ranges are obtained:

-   -   for the composition at 50° C.: 13 to 100 A/dm²     -   for the composition at 55° C.: 16 to 100 A/dm²

There also, these results are particularly good.

EXAMPLE 5

In this example, the influence, on the ageing of the bath, of the glycine concentration in the composition is studied. Three compositions are tested: a concentration equal to 0.75 ml/l (composition B1), a concentration equal to 1 mol/l (composition B2) and a concentration equal to 1.25 mol/l (composition B3).

For substrates as described in Example 1, a method according to the conditions described in Example 1 is implemented, with the exception of the value of the pH, which is equal to 1, and the glycine concentration, which is equal to 0.75 mol/l for composition B1, to 1.00 mol/l for composition B2 or to 1.25 mol/l for composition B3.

For each composition B1, B2 and B3, the following experiment is carried out.

For each substrate, a current density of 40 A/dm² is imposed between the substrate and the anode for a suitable period for forming on the surface of the substrate a chromium coating 50 μm thick, which, for each coating, corresponds to a quantity of electrical load imposed per volume of aqueous composition of between 2.2 and 2.3 Ah/L. A plurality of substrates are thus treated successively in the same bath at various stages of ageing, until an ageing of the bath of 33.6 Ah/L is reached.

After this treatment, each substrate is subjected to a degassing step for 3 h at 190° C.

For each of the compositions, with the new bath, homogeneous, metallic and adherent deposits are obtained. For each of the substrates treated, the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.

FIG. 4 shows photographs of the substrates thus obtained, respectively at a/ for composition B1, at b/ for composition B2 and at c/ for composition B3.

As can be observed, for composition B1, according to the invention, a loss of adhesion of the metallic coating (indicated by a white arrow on the figure) is observed at the region rubbed with abrasive paper for the metallic coatings that were formed in the electrolytic baths in which the equivalent of 24.6 Ah/L and more has been imposed. For the baths in which the equivalent of 22.4 Ah/L or less has been imposed, the metallic coating remains adherent.

For composition B2, the loss of adhesion of the coating is observed at much shorter ageing times, as soon as after 15.6 Ah/L (loss of adhesion indicated by a black arrow).

For composition B3, the loss of adhesion occurs after having imposed in the bath an even lower electrical load, equivalent to 6.8 Ah/L.

This demonstrates that, entirely surprisingly, the electrolytic baths based on compositions containing 1 mol/l of glycine and more have a greatly reduced service life compared with baths formed from compositions according to the invention containing no more than 0.9 mol/l of glycine. The latter advantageously have a long service life, during which they make it possible to form, on the surface of the substrate, a metallic chromium coating having good adhesion. 

1. An aqueous liquid composition for the electrolytic deposition of a chromium coating on the surface of a substrate, said composition containing 0.4 to 0.9 mol/l of trivalent chromium salt, 0.6 to 0.9 mol/l of glycine, an alkali metal salt, an aluminium salt and optionally an ammonium salt, and having a pH of between 0 and
 1. 2. The composition according to claim 1, wherein the concentration of trivalent chromium salt is between 0.7 and 0.9 mol/l.
 3. The composition according to claim 1, wherein at least one of said trivalent chromium salt, said alkali metal salt, said aluminium salt and, where applicable, said ammonium salt, is selected from a group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, sulfonates, thiocyanates, or any one of the mixtures thereof.
 4. The composition according to claim 1, wherein the concentration of aluminium salt is between 0.06 and 0.7 mol/l.
 5. The composition according to claim 1, wherein the concentration of alkali metal salt is between 0.2 and 1.9 mol/l.
 6. The composition according to claim 1, wherein the concentration of ammonium salt is between 0 and 1.0 mol/l.
 7. The composition according to claim 1, wherein the concentration of ammonium salt is between 0.5 and 0.8 mol/l and the concentration of alkali metal salt is between 0.2 and 1.3 mol/l.
 8. The composition according to claim 1, wherein the composition is devoid of ammonium salt and the concentration of alkali metal salt is between 1.5 and 1.9 mol/l.
 9. A method for the electrolytic deposition of a chromium coating on the surface of a substrate, comprising: immersing said substrate and anode in a bath of an aqueous composition according to claim 1, and applying a current between said substrate and said anode.
 10. The method according to claim 9, further comprising applying a continuous current between the substrate and the anode.
 11. The method according to claim 9, wherein the temperature of said bath is between 20° C. and 80° C.
 12. The method according to claim 9, wherein applying a current between the substrate and the anode is carried out for a suitable duration for forming on the surface of the substrate a chromium coating with a thickness of between 5 and 500 μm.
 13. The composition according to claim 3, wherein a plurality of said trivalent chromium salt, said alkali metal salt, said aluminium salt and, where applicable, said ammonium salt, are selected in the group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, sulfonates, thiocyanates, or any one of the mixtures thereof.
 14. The composition according to claim 4, wherein the concentration of aluminium salt is between 0.2 and 0.3 mol/l.
 15. The method according to claim 10, wherein the density of said continuous current is between 10 and 100 A/dm2. 